This invention relates to techniques for measuring a displacement using a magnetic field source and a sensor disposed within the magnetic field. The invention is applicable in any field permitting the generation of a magnetic field and the taking of measurements as specified herein. Though not limited to any specific field, the invention has particular use in subsurface measurements.
Accurate borehole dimension data are important for both the measurement of earth formation parameters penetrated by the borehole and completion of the well after drilling. Borehole dimension data from successive trips in the borehole can be used to monitor wellbore conditions such as early indications of borehole washout and impending wellbore instability. Information regarding dimensions can allow a driller to take remedial actions during the drilling operation to prevent damage loss of the borehole or drilling equipment. As shown in FIG. 1, once a borehole is drilled to the desired depth, it is typically xe2x80x9ccompletedxe2x80x9d with a steel casing (1) around which cement (2) is pumped thereby filling the casing-borehole annulus. Information regarding the dimension of, for example, the radius of the borehole is very useful in determining completion requirements, such as the amount of cement required to properly fill the annulus. Also, prior art logging-while-drilling (LWD) or measurement-while-drilling (MWD) systems, like their wireline counterparts, are sensitive to borehole size. Accurate dimension information is required to properly correct parametric measurements from these systems. Measuring the dimensions of a borehole, such as its radius, can be accomplished by several methods. U.S. Pat. No. 6,285,026 issued to Evans et al. is directed toward the determination of radial dimensions of a borehole by irradiating the formation with neutrons and measuring the neutron flux within the borehole.
Also known in the art are systems that measure the diameter of a borehole with a caliper tool, which uses a sensing element such as a potentiometer. Typically, these devices use spring-loaded arms to contact the borehole wall. The arms are connected to one or more elements of a potentiometer for determining the instantaneous borehole diameter. These types of caliper systems, because they are exposed to harsh downhole conditions, require pressure bulkhead and pressure sealing components to protect their parts.
There remains a need for improved borehole measurements techniques and tools that can better withstand the downhole environments.
The invention provides an apparatus for measuring a radius of a hole. The apparatus includes a support member having at least one arm, each at least one arm having a first end and a second end, the first end attached to the support member; a magnet moveably attached to the support member via the second end of each at least one arm; and a first magnetic sensor fixed to the support member for measuring a magnetic field of the magnet.
The invention provides a downhole tool for measuring a radius of a borehole. The tool includes a housing having an exterior surface and an interior; at least one arm, each of the at least one arm having a first end and a second end, the first end attached to the exterior surface of the housing; at least one magnet moveably attached to the exterior surface of the housing and each at least one magnet attached to the second end of each at least one arm; and a first magnetic sensor, fixed in the interior of the housing for measuring a magnetic field of the at least one magnet.
The invention provides a method for measuring a displacement. The method includes generating a magnetic field using a magnetic field source; measuring a first magnitude of a component of the magnetic field using a sensor disposed within the magnetic field; displacing the magnetic field source; measuring a second magnitude of the component of the magnetic field using the sensor; determining a distance that the magnetic field source is displaced using the first magnitude and the second magnitude.